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Here is a code book question. Maybe a good laugh too. I live in New York State. I have a builder friend who is installing two propane fireplaces in his McMansion. He wants to install a return in the ceiling above said fireplaces and use his air handler in manual to circulate the 'free' heat throughout the house. Now keep in mind that there is 1 A/H for each of 2 floors and the A/H primary purpose is for A/C. I suggested that installing a hydro coil or a resistive heating element would be more economical, practical and safer than trying to Rube Goldberg this idea and try and capture the heat. Also, he is telling me that he wants to install a 14x30 register, (on the first floor) slap in a 7" flex and run this thing up a wall to the return, in the attic. Also making some kind of monster return so that it also feeds the 2 upstairs air handlers to put some heat up stairs as well. I understand what he is trying to do, but I don't think that it is practical, necessary or viable economically with all the work that is involved when weighed in with the expense versus benefits.
I think that the guy has a good idea, just not practial in my opinion or experiance.

We have customers who like to run the gas fireplace for effect. They tell me it gets hot in the room with the fireplace. I tell them to turn the fan on for the airhandler. This draws the hot air out of that room and distributes it through out the house. It seems to work fairly good and the end roomsdo not get super cold, more even temps through out the whole house.

If it's not a big deal to put the return where he wants it, I say do it. It can't hurt anything (or could it?)

what type of Fp?

If these are direct vented fireplaces, the use of forced air ventilation should have no impact on them unless it depressurizes the room to >25 Pascals negative.

However, if these are vented logs in an open fireplace or a B-vented fireplace, you will backdraft the Fp and kill or sicken everyone in the house! It could also depressurize the room enough to cause cold air infiltration around the Fp. He'd better spec. that that room be tested for balance using a flow hood.

Can you provide the make and model Fp?

If he can afford a McMansion, why is he so desperate to control a few lousy BTUs? It ain't like he's gonna heat the house with it. The heat is not being lost to the outside so its only controlling where that heat goes. The best way to do that is with a normal balanced, sealed, functional duct layout and air handler. You say he has 2 air handlers upstairs. That's 2 machines sucking the air out of the house, doing something to it, and hopefully pumping the same amt. of air back into the home. If the supply side has any holes in it, this will become a massive depressurization source, not to mention the inefficiencies from using flex duct and trying to insulate a unit that is outside the thermal envelope of the house. Even when the system isn't running, it will act as a huge hole in the room futher exacerbating the depressurization downstairs.

The fire codes may not permit this "chimney" running from a Fp to the attic unchecked.